(1) Field of the Invention
The present invention relates to a digital recording and reproducing apparatus in which relatively long recording as well as a multiple number of search playback modes having different search speeds can be realized by compressing band width of the recording signal.
(2) Description of the Prior Art
In order to make an international agreement of standard specifications of a digital VTR for home-use that is expected as the one for a next generation, the HD-DIGITAL VCR CONFERENCE has been set up and an agreement was reached internationally as to between the recording schemes of current television system (to be abbreviated, hereinafter, as SDTV (Standard Definition)) and that of the HDTV system in April, 1994. The specifications thus agreed are characterized that both the television signals for the SDTV and the HDTV can be recorded using a common configuration. This method mainly effects high-efficiency coding inside frame. Specifically, the technique mainly performs the discreet cosine transform (to be referred to, hereinafter, as DCT) and the variable-length coding. One of reasons the inside-frame high-efficiency coding technique was adopted is that there is a need for high-speed searching reproduction which makes it easy to effect editing operations. That is, the system should reproduce high-quality pictures without unnaturality when pictures are reproduced in a search mode of at least ten times or less speed.
FIG. 1 is a block diagram briefly showing the above-stated home-use DVCR which has been internationally consented (those of SD.VCR specifications will hereinafter be referred to as a SD-VCR). An inputted image signal is A/D converted into the luminance signal Y and two kinds of chrominance signals C.sub.N and C.sub.W and then divided into blocks of 8.times.8 pixels. Thereafter, the data is shuffled for every block in a block shuffling section 101. This is done both to disperse frequency components so as to improve the efficiency of the following high-efficiency coding mainly consisting of the DCT and to disperse burst errors to be caused by dropout at the playback mode. A high-efficiency coding section 102 effects an orthogonal transform using the DCT technique so that the signals are represented by coefficients with respect to frequency components. The section 102 further effects the quantization of the coefficients adaptively as well as the variable-length coding to remove the redundancy or consecutive 0's. Sufficient removal of redundancy in the high-efficiency coding section 102 markedly reduces the bit rate of the signal. In an error correction coding section 103, the thus high-efficiency coded, compressed signal is added with a necessary parity code for correcting code errors which would be generated at the playback mode. In a Sync and ID adding section 104, a synchronizing code for effecting PCM synchronization and an ID code for discriminating the block content are added to each of sync blocks containing the synchronizing code. A modulating section 105 represents a modulator for efficiently recording the recording signal. The 24-25 modulating method is adopted in the modulator used in the configuration of the DVCR specifications for the purpose of reducing the d.c. component. The output from the modulation is amplified through a recording amplifier and recorded on a magnetic recording medium via a video head.
At the playback mode, the recorded signal is picked up via the video head, and the thus regenerated signal is amplified through a regenerating amplifier and supplied to a demodulating section 107, to thereby recover digital information. Then, the operation is effected in just the reverse direction or in the opposed direction to that effected in the recording mode. That is, a sync and ID detecting section 108 detects the synchronizing code for PCM as well as decodes and deciphers the content of the ID code. An error correcting and decoding section 109 detects code errors and completely corrects the errors if any. A decoding and modifying section 110 subjects the compressed video information by the high-efficiency coding section to the variable-length decoding and the inverse quantization and effects IDCT for the thus processed video information to recover a video signal approximately corresponding to the original video signal. If there is an erroneous code which is unrecoverable, the section 110 effects interpolation using data before and after the code in question. The thus recovered output is still not the complete video signal, but should be deshuffled by every block in the following deshuffling section 111, to thereby reproduce the original video signal.
FIG. 2 shows a structure and a recording format of sync blocks in the DVCR stated above. Each sync block is composed of 90 bytes containing two bytes for a synchronizing code, three bytes for an ID code and 77 bytes for video data with eight bytes of inner parity of the Reed-Solomon correcting code. Assigned for video data in the format are 135 sync blocks (to be abbreviated SB, hereinbelow) each having 77 bytes.
At present, no specific scheme is established for recording the signal which is formed by compressing image data on the basis of the high-efficiency coding scheme standardized as MPEG-2 by the MPEG (Moving Picture Image Coding Expert Group) using the tape driver, signal processor and recording and reproducing system in the aforementioned DVCR. The MPEG-2 coding scheme was first defined in the MPEG-2 System Committee Draft (ISO/IEC JTC1/SC29/WG11/N0601, November, 1993) and then discussed at the Grand Alliance Conference of Apr. 14, 1994. The MPEG-2 standard was approved Nov. 11, 1994 by the International Organization of Standards (ISO), and is now formally referred to as ISO 13818. FIG. 3 shows a structure of coding proposed as a provisional standard for the ATV. In the figure, a symbol I represents a coding process within a frame, P represents a predictive coding process relative to a forward frame as an interframe coding process and B represents another predictive coding process relative to both the forward and backward frames. In the case where the video signal thus formulated based on the above interframe predicative coding is recorded as it is on the basis of the SD specifications already consented, the signal is regenerated from several different tracks as disconnected data as shown in FIGS. 4A and 4B when the system is operated in the picture search playback mode. Accordingly, it is barely possible to reproduce a complete picture with clear content.
In a case where the MPEG signal processing scheme is adopted as in the ATV, noting the fact that intraframe processed I-pictures repeatedly appear every twelfth frame, a technique in which special data prepared for the special playback is recorded in specially allotted recording areas has been disclosed in a Technical Report vol.17 No.59 of The Institute of Television Engineers of Japan and in a Technical Report MR93-28(1993-10) of The Institute of Electronics, Information and Commutation Engineers. This method however requires a considerable amount of data for reproducing I-pictures for the ATV as will be shown hereinbelow.
Consider a case where I-pictures are recorded with a reduced quality of image equivalent to that of the NTSC or less, for example, where only DCT coefficients for d.c. components which exist one for each DCT block of 8.times.8 pixels are used. In this case, suppose that the number of valid samples is 1,920; the number of valid scan lines is 1,080 therefore the blocks amounts to 32,400. When only the d.c. components which exist one for every 64 components of each DCT are transformed into 8-bit data, the amount of data to be recorded totals to 32,400 bytes. In addition, if four bits are allotted for each of the remaining coefficients for a.c. components, another data of 1,020,600 bytes is required since each DCT block has 63 a.c. components. In other words, use of the a.c. coefficients bulks required data amount up to about 32 times. On the other hand, actually available data amount in the search playback mode depends on the recording system and the type of the recorded signal used. As an example of the ATV, if transmitting packets are transmitted at a rate of 19.3 Mbps and the rotating rate of the head is set up to be 150 rps, each track needs to have a data recording area of 105 SBs in order to completely record the transmitting packets. Accordingly, 30 SBs can be allotted as the data recording area for data in the search playback mode and therefore the total allowable data-recording area for the I-pictures of the ATV signal periodically appearing every twelfth frame amounts to 3600 SBs. Recording only the d.c. components of the I-picture requires 421 SBs, which corresponds to about one-eighth of the allowable data-recording area while recording of eight a.c. coefficients for every DCT block requires further 3366 SBs.
If data-recording areas able to be commonly used for the two kinds of head arrangements are to be considered as will be described later with reference to FIG. 14, sixty data-recording areas each consisting of 60 SBs are required within a period of twelve frames for the triple(3.times.)-speed search-playback mode. Of these, forty areas can be effectively used in the aforementioned search-playback mode and this corresponds to 2,400 SBs or 184,800 bytes. In this case, the allowable number of a.c. components for each DCT block is 4.7. This is the result when all the areas are allotted to 3.times.-speed search-playback mode. In practice, data-recording areas should be allotted for quintuple(5.times.)-speed mode, fifteen-times(15.times.)-speed mode etc., so that effective data amount allowable for each of the search modes decreases considerably. For example, suppose that 1,740 SBs can be taken as allowable recording areas for 3.times.-speed search mode, the available recording-areas amount to 1,160 SBs or 89,320 bytes. Further details will be discussed later.
Particularly, in realizing a high-speed search mode, (for example, 15.times.-speed search mode), to obtain ten effective recording areas within 30 tracks requires 96 established recording areas. This will be described later with reference to FIG. 12.
The following items are problems to be solved as to the recording apparatus described heretofore:
(1) As future broadcasting, both the ATV broadcasting for high-quality images and the SDTV (NTSC, PAL or SECAM) broadcasting are planned to be delivered in parallel through ground broadcasting, satellite broadcasting and CATV. However, there is no apparatus which is able to record both the high-bit rate signal and the low-bit rate signal, therefore, two types of recording apparatuses are required for recording respective signals.
(2) Since the conventional apparatus has no means for recording the low-bit-rate signal simultaneously with the high-bit-rate signal so that the low-bit-rate signal can be used for the normal playback and the special playback, it is necessary to display special-play images using a high-priced, high-resolution, wide display even if quality of pictures reproducible in the search mode is lower than that of the SDTV.
(3) Since, in the special-playback method already proposed as to the MPEG recording and as to the ATV recording, only the I-pictures are to be recorded and used for the special playback operation, it is impossible to reproduce smoothly animated images.
(4) There is a report of a method whereby intraframes and interframes are separately recorded. Since the data amount of an intraframe is, in general, ten times greater than that of an interframe, this makes it very difficult to establish recording areas for special play. There is another proposal that interframes are converted into intraframes for special playback, but this method still increases the amount of data required for special play.
(5) If the frame cycle of the recorded signal is not related with the multiple number of recording areas, it becomes difficult to meet recording positions of the normal playback data with recording positions of the special playback data. This makes it difficult to make sure the content of data on the tape in the search-playback mode and makes the signal processing circuit complicated.
(6) Mere allocation of recording areas for special playback bulks the amount of data to be recorded if there is necessity to ensure both forward and reverse searching operations.
(7) As to the head arrangement, in order to establish the interchangeability between the use of the double head and the use of the single head, greater recording areas is necessary.
(8) Since in the search-playback at a high speed, the periodic cycle of retracing with the head becomes long, it is impossible to actively follow the change of the search-playback speed. In order for the system to realize the active follow, duplicated data must be recorded in different places. This requires greater areas for data recording.